Conventionally, a membrane separation active sludge treating method has been known as a method of treating wastewater such as sewage, industrial wastewater, household wastewater and others (see, for example, patent documents 1 to 3).
In this membrane separation active sludge treating method, an immersion type membrane separation apparatus is immersed and installed in a reaction tank for treating wastewater by active sludge treatment. This immersion type membrane separation apparatus is composed by disposing a plurality of membrane cartridges of organic flat membrane type in a main body support frame, and filling by disposing the membrane surface along in the vertical direction. By making use of a negative pressure (inter-membrane differential pressure) generated in the membrane cartridges from the membrane surface between the membrane cartridges, wastewater is sucked and filtered, and the filtered water is taken out as a treated water.
Moreover, an aeration device is disposed in the bottom of a reaction tank at a lower position from an immersion type membrane separation apparatus immersed in the reaction tank, and the membrane cartridges disposed in parallel are agitated within the tank by generating an ascending stream in the wastewater by elevating a membrane cleaning gas (generally, air), so that the surface of the membrane cartridge may be cleaned.
Although the membrane cleaning gas discharged from the aeration device becomes foams and float between membrane cartridges disposed in parallel, thereby cleaning the contaminants sticking to the membrane surface of each membrane cartridge, the floating force of the membrane cleaning gas and the ascending stream of wastewater generated at the same time work to apply vibrations to the membrane cartridges.
Such vibrations of the membrane cartridges are particularly violent in the upper parts of the membrane cartridges. As a result, a strong friction occurs among the frame for mounting the membrane cartridges, the upper parts of membrane cartridges, and the membrane cartridges, and the membrane cartridges are worn significantly, and the service life of the membrane cartridges may be shortened extremely.
Moreover, the membrane cartridges disposed adjacently in parallel at specified intervals are connected with treated water discharge pipes for discharging the treated water filtered on the membrane surface from the membrane cartridges. Hence, solid matter sticks to the treated water discharge pipes at the upper position of the membrane cartridges, and the flow velocity of the treated water ascending between the membrane cartridges is suppressed, and the treated water volume passing through the membrane cartridges is lowered.